Indeed. B&B are also guilty of using similarity scores to gauge a player's career. It simply tells you what kind of players they had similar "styles" to, the "shape" of their statistics essentially. It's not a good indicator for the "size" of their statistics.

WAR is a pretty good number for evaluating HoF.
(Using baseball-reference.com numbers here).
100% of 53 eligible players with 73+ career WAR are in the HoF.
70% of 64 eligible players with 60-72 career WAR are in the HoF.
51% of 91 eligible playes with 50-59 career WAR are in the HoF.

And then you want to look at how many "great" seasons they had. As a rough guide, 5+ WAR is considered all-star quality, 8+ WAR is MVP quality.

Ron Santo has 66.6 WAR, over 15 seasons. 7 seasons of 5+. 3 seasons of 8+.
Joe Morgan has 97.1 WAR, over 22 seasons. 8 seasons of 5+. 5 seasons of 8+.

I think that puts Santo comfortably above "borderline" status, albeit not in sure-thing territory. There are only 6 players "better" than him that are not in the Hall. Joe Morgan, even with the benefit of 7 more seasons, definitely had a better career. 5 MVP-caliber seasons.

And here's the players with the most WAR not yet admitted:
72.7 Jim McCormick
71.4 Lou Whitaker
70.9 Bill Dahlen
69.7 Larry Walker
67.3 Bobby Grich
67.1 Alan Trammell
66.2 Tim Raines
66.2 Rick Reuschel
66.1 Rafael Palmeiro
64.4 Edgar Martinez
64.3 Kevin Brown